Fuel cell stacks, which are composed of individual fuel cells aligned electrically in series, produce electricity via an electrochemical reaction between hydrogen and oxygen. Within each fuel cell, the hydrogen contacts an anode catalyst where it is converted to hydrogen ions and free electrons. The hydrogen ions migrate across an electrolyte to a cathode catalyst where they react with oxygen and the free electrons which have passed through an external load, thereby producing electricity and forming water.
During phosphoric acid fuel cell stack operation, for example, electrolyte migrates both intra-cell and inter-cell due to an electrical potential gradient across individual cells and across the cell stack, thereby decreasing the volume of electrolyte in one area of the fuel cell while increasing it in another. Not only does the electrolyte migrate from the cathode side electrolyte reservoir plate within one cell of a stack to the anode of that cell, it may also migrate across the separator plate to the next cell of the stack. This intra-cell migration can cause electrolyte flooding of the anode, thereby reducing the anode performance and the cell efficiency, while inter-cell migration can cause electrolyte flooding of end fuel cells in a fuel cell substack (approximately 8 fuel cells which are typically part of a larger cell stack), thereby reducing performance of the stack and stack efficiency.
Prevention of electrolyte migration has been addressed in various fashions including the utilization of a graphite separator plate (see U.S. Pat. Nos. 4,360,485 and 4,301,222), a fluoropolymer barrier, and a separator plate comprised of a graphite-fluoropolymer mixture. Since individual fuel cells are typically stacked such that the first and last component of the stack is a porous graphite electrolyte reservoir plate and/or porous graphite cooler holder (herein after referred to as porous component), the migration prevention devices are located between individual cells, adjacent to the porous components of these cells.
These conventional migration prevention devices were only partially successful. The graphite separator plate was meant to block inter-cell electrolyte migration. However, due to the graphite separator plate's porosity of up to about 25%, the plate functioned as and electrolyte absorber. Electrolyte which passed from the porous component was absorbed into the plate, thereby preventing the electrolyte from migrating to the next cell of the stack. However, once the absorption capacity of the plate had been attained, typically after about 28,000 hours of fuel cell operation and occasionally after only about 3,000 hours, the plate no longer inhibited migration.
Unlike the graphite separator plate, the fluoropolymer barrier did function as a blockade to electrolyte migration because it was substantially solid. However, after about 500 hours of fuel cell operation, this barrier significantly degraded due to the difference in the coefficient of thermal expansion of the fluoropolymer and the graphite of the porous component, about 75.times.10.sup.-6 ppm/.degree.F. versus 1 ppm/.degree.F., respectfully. Thermal cycling of the fuel cell caused the fluoropolymer barrier to pull away from the porous component, thereby forming pathways which allowed a relatively large amount of electrolyte migration, typically about 150 to about 2,500 g/ft.sup.2/ 1,000 hrs. (grams per square foot per 1,000 hours).
Finally, the graphite-fluoropolymer mixture functioned similar to both the graphite plate and the fluoropolymer barrier since the porosity of the plate was reduced with fluoropolymer which blocked the passage of the electrolyte which entered the plate. This mixture initially prevented electrolyte migration due to its low porosity, about 15%. However, as thermal cycling of the fuel cell caused crevices to form in the mixture, pathways developed which allowed electrolyte migration. Therefore, although the mixture was an improvement over the fluoropolymer barrier since the graphite in the mixture acted as a blockade thereby decreasing electrolyte migration, it still failed to maintain electrolyte migration below about 0.5 g/ft.sup.2/ 1,000 hrs. for the typical life of a fuel cell, about 40,000 hours.
Referring to bars 1-4 of the FIGURE, the graphite separator plate (bars 3 and 4) minimized electrolyte migration to about 0.09 to about 26 g/ft.sup.2/ 1,000 hrs., while the fluoropolymer barrier and the graphite-fluoropolymer mixture experienced electrolyte migration of about 120 to about 2,500 g/ft.sup.2/ 1,000 hrs. (bar 1) and about 20 to about 100 g/ft.sup.2/ 1,000 hrs. (bar 2), respectively. However, in order to attain an acceptable fuel cell current density of about 200 ASF for about 40,000 hours, the electrolyte migration should be below about 0.5 g/ft.sup.2/ 1,000 hrs.
Consequently, what is needed in the art is a separator plate capable of minimizing inter-cell electrolyte migration to below about 0.5 g/ft.sup.2/ 1,000 hrs. (assuming a linear rate) for at least about 40,000 hours of fuel cell operation.